Some communication systems, such as a multi-link “top-of-rack” Ethernet switch, include a main communication controller, sometimes referred to as a “switch chip,” which is typically connected to several receptacles in which optical modules may be plugged. Depending on the particular implementation, the communication controller may be configured to communicate with any number of such modules using a suitable communication interface. One such interface is the 100 Gigabit Attachment Unit Interface-4 (CAUI-4) chip-to-module interface, which is defined by the Institute of Electric and Electronics Engineers (IEEE) P802.3bm specification. The CAUI-4 chip-to-module interface enables communication between chips and modules at a data rate of up to 100 gigabits-per-second using four lanes in parallel with 25.78 gigabaud signaling on each lane. Of course, transmitting data between chips and modules at such high baud rates requires robust signal integrity, which limits the distance between the chip and respective module to a few inches to meet the low bit error ratio (BER) required by the CAUI-4 chip-to-module interface specification.
To facilitate the placement of modules at a distance greater than the typical limited distance, the CAUI-4 defines a chip-to-chip interface that facilitates the use of retimer chips to extend the overall distance between the communication controller (e.g., the switch chip) and associated module. To increase the distance even further, multiple retimers may be used in series. The increase in overall distance is accomplished by specifying transmit-side equalization in both sides of the CAUI-5 chip-to-chip link. For example, the IEEE P802.3bm, Annex 83D, Draft 2.1, specification defines the transmitter equalization as a three-tap feed forward filter having filter coefficients selected from a few possible combinations. However, the IEEE P802.bm specification does not define any specific functionality for selecting or controlling the equalization settings. Although some communications systems allow for partner-to-partner in-band back channel communication to facilitate the tuning of the filter coefficients, such in-band, back channel back channel communications is complex and can disrupt system performance.